Solid oxide fuel cells and related devices

ABSTRACT

A solid electrolyte fuel cell component is formed by tape casting an electrolyte layer and electrode layers to form a green tape which can be manipulated. The green tape is coiled into a form having an S-shape central portion having oppositely-directed loops, so as to provide a first longitudinal channel presenting an anode surface and a second longitudinal channel presenting a cathode surface. After coiling, the assembly is fired to produce a solid, sintered product.

[0001] This invention relates to solid oxide fuel cells, and to devicessimilar to fuel cells for use in electrocatalysis and electrolysis ingas based processes.

[0002] Despite considerable research and development effort, fuel cellshave not yet been successfully commercialised. Gradual progress has beenmade in developing solid oxide fuel cells in two basic arrangements,flat plate and tubular but costs remain high and there are sealing andinterconnect problems.

[0003] The present invention seeks to provide a radical means ofaddressing these problems.

[0004] The present invention provides, in one aspect, a method of makinga component having an anode, a cathode and a solid electrolyte, themethod comprising using tape casting to produce a green tape which iscohesive but flexible and firing the green tape to produce a rigidcomponent; the green tape comprising at least three layers each of whichis derived from a respective slurry comprising metal/ceramic particlesdispersed in a carrier liquid.

[0005] From another aspect, the invention provides a component for usein a fuel cell or an electrochemical device, the component having agenerally elongate tubular form divided by a central web into twochannels, one of the channels presenting an anode surface to materialflowing therethrough, and the other channel presenting a cathode surfaceto material flowing therethrough, the component further comprising asolid electrolyte between said anode and cathode.

[0006] The invention further provides fuel cells comprising componentsin accordance with, or made by the method of, the invention.

[0007] Preferred features of the invention and its advantages will beapparent from the following description and claims.

[0008] Embodiments of the invention will now be described, by way ofexample only, with reference to the drawings, in which:

[0009]FIG. 1 illustrates the construction and operation of a known typeof fuel cell;

[0010]FIG. 2 is a schematic side view showing an apparatus used for tapecasting;

[0011]FIG. 3 is a similar view of an apparatus used in the invention;

[0012]FIG. 4 is a schematic perspective view of a fuel cell component inaccordance with the invention;

[0013]FIG. 5A is a side view of a modified form of fuel cell component;

[0014]FIG. 5B is a side view of the modified component following a firststep to produce a seal at one end; and

[0015]FIGS. 5C and 5D are side and plan views, respectively, of thecomponent following a second step.

BACKGROUND

[0016] Referring to FIG. 1, a solid oxide fuel cell comprises an anode10, a cathode 12, and a solid electrolyte 14. The cell produceselectricity by electrochemically combining hydrogen (which may bepresent as such, or in a hydrocarbon fuel) and oxygen (which may bepresent as such or in air). The oxygen is reduced at the cathode 12,accepting electrons from the external circuit to form O²⁻ ions (equation(1)) which are conducted through the solid electrolyte 14 to the anode10. At the anode/electrolyte interface, hydrogen is oxidised to formH₂O, releasing electrons back into the external circuit (equation (2)).

O₂+4e⁻⇄2O²⁻  (1)

2H₂+2O²⁻⇄2H₂O+4e⁻  (2)

[0017] Each of the three components must not react with any othercomponent it is in contact with, must be stable at operatingtemperatures, and all three must have similar thermal expansions. Theanode 10 and cathode 12 need high electronic conductivity and sufficientporosity to allow the gases to reach the electrode/electrolyteinterface. In comparison, the electrolyte must be dense, preventing gasflow, have high oxygen ion conductivity, allowing O²⁻ ions to permeatewith minimum resistance, and as small an electron transport number aspossible.

[0018] One known family of fuel cells uses yttria stabilised zirconia(YSZ). The anode consists of YSZ mixed with Ni, and the cathode of YSZmixed with Sr doped LaMnO₃. This serves to obtain similar thermalexpansion to the electrolyte, and also acts to increase the triple phaseboundary (the area of contact between anodic/cathodic material,electrolytic material, and the gas phase).

[0019] Two main types of fuel cell exist at present. One is the planarcell, in which flat plates in the geometry shown in FIG. 1 are stackedone on top of another separated by an interconnect. The other istubular, in which the materials are formed into tubes with the insidesurface cathode and the outer surface anode. Air and fuel (hydrogensource) are passed over the corresponding electrodes.

Preferred Embodiments

[0020] Turning to FIG. 2, the present invention makes use of a processof tape casting to form the electrode and electrolyte structures. Tapecasting as a process is known per se, see for example ‘Tape CastingTheory and Practice’ by Richard E Mistler and Eric R Twiname, but haspreviously been used in the field of fuel cells only to manufacturesingle layers such as anodes or cathodes.

[0021] Tape casting is the production of thin sheets of ceramic and/ormetallic material. The ceramic/metallic powders are mixed by ball milltogether with various organic materials: solvent, dispersing agent,binder and plasticizer which hold the individual particles in ahomogeneous distribution throughout the slurry.

[0022] As seen in FIG. 2, the slurry 20 is cast onto a moving carriersurface 22 by a doctor blade 24. The carrier surface 22 may suitably bea glass plate or Mylar sheet. Upon evaporation of the solvent, aflexible ‘green’ tape is produced which may be handled and manipulated.The green tape is subsequently fired, removing the remaining organicmaterial and producing a hard, rigid sintered material.

[0023] The ball milling stage is important to ensure that all the softagglomerates are broken down and the powder is well dispersed. The ballmilling is normally performed on the powder, solvent and dispersant; thebinder and plasticizer added subsequently, and the entire mix mayundergo further ball milling but at a slower speed. De-airing the slurryand maintaining a constant casting speed ensure constant thickness andsmooth surface finish of the green tapes.

[0024]FIG. 3 shows an apparatus in which three slurries 20 a, 20 b, 20 care cast sequentially on a single carrier surface 22, thus producing athree-layer green tape which can be handled as a single unit and firedto produce a rigid unitary structure. By using suitable materials in thethree slurries, a fuel cell component comprising anode, cathode andsolid electrolyte is produced. A preferred composition is: anode YSZ andNiO which is reduced to Ni under fuel conditions cathode YSZ and Srdoped LaMnO₃ electrolyte YSZ (8-10 mol % yttria, balance zirconia)

[0025] One alternative to the multiple casting arrangement of FIG. 3 isas follows. The electrolyte layer is deposited first, and one electrodelayer is deposited on top, once the electrolyte layer has partiallydried. This composite is allowed to dry somewhat, after which thetwo-layer composite is turned over and the second electrode layerdeposited on top.

[0026] Another alternative is to produce three separate ribbons by tapecasting, and combine these by stacking and applying pressure, forexample by passing between rollers. This has the advantage of furtherreducing the electrolyte thickness.

[0027] The three layer structure produced by any of the foregoingmethods forms a single component which can be handled and fired as aunit (co-fired). This contrasts with prior art use of tape casting,where each electrolyte or electrode layer is formed and firedseparately.

[0028] These fuel cell components can be produced simply by tape castingand firing, resulting in flat plate components. However, the inventionalso provides a novel form of fuel cell which is made possible by theuse of tape casting.

[0029] Referring to FIG. 4, a three layer tape having anode 40,electrolyte 42 and cathode 44 is wound while in the green state prior tofiring. The winding is such as to produce oppositely-directed loops inan S-shape in the centre of the component, thus forming longitudinalchannels 46 and 48 separated by a central web 50. One channel 46 has asurface of anode material 40, while the other channel 48 has a surfaceof cathode material 44. Typically, the overall cross-section of thewound component may be about 50 mm, and the channels 46 and 48 each havea width of about 5 mm. The component may be wound from a tape 0.2 m×2 m.

[0030] In use, air is passed through the channel 48 to contact thecathode 44, and hydrogen (or a hydrogen-containing fuel) is passedthrough the channel 46 to contact the anode 40. The anode and cathodeare porous, preferably about 50% porosity, and thus the air and hydrogenpermeate through the anode and cathode layers and are not simply incontact with the parts fronting the channels 46 and 48.

[0031] The arrangement shown in FIG. 4 thus provides a fuel cellcomponent which is simple to make, gives a large active area withincompact dimensions, and combines the best features of flat plate andtubular fuel cell geometries.

[0032]FIG. 5 illustrates a modification of the embodiment of FIG. 4.This makes use of the fact that the electrolyte layer 42 is dense andimpermeable. In FIG. 5, the electrolyte layer 42 is of greater widththan the electrode layers 40 and 44 and thus forms projecting portions42 a, 42 b when the layers are wound or coiled. The projecting portion42 a is pressed (FIG. 5A) to form a flattened end (FIG. 5B) which isthen turned over (FIGS. 5C and 5D) to form a seal, in the manner of atoothpaste tube. The assembly is then fired to form a rigid componentsealed at one end.

[0033] The projecting portion 42 b at the other end may be used forconnecting the component to a gas supplies such as fuel and airmanifolds.

[0034] Choice of Materials

[0035] The foregoing embodiment is based upon the use of YSZ materials.Such materials are presently preferred in carrying out the invention,and it is believed that the use of high-zirconia materials will be ofparticular benefit when using co-firing of multiple tape layers.However, other materials may be used in implementing the invention.

[0036] The electrolyte should be an ionically conducting oxide capableof transporting either oxygen ions or protons or both. Typical materialsin addition to yttria-zirconia are scandia-stabilised zirconia, ceriumoxide based materials, lanthanum gallate materials, and oxide protonconductors such as barium cerate, strontium zirconate, and otherperovskites based on cerium, niobium or zirconium, and titaniumcontaining alkaline earth strontium or barium or rare earths or yttriumor scandium.

[0037] Alternative air electrode materials would be based on lanthanumstrontium cobaltate, lanthanum strontium iron oxide, and variouscombinations of manganese cobalt and iron in the same perovskitelattice.

[0038] The fuel electrode in addition to nickel zirconia cermets may usecopper zirconia cermets, copper ceria cermets, nickel ceria cermets,perovskites based on lanthanum chromate, and fluorites based on yttriazirconia titania either on their own or in combination with a currentcollecting material.

[0039] In summary, the invention may be applied to any oxide fuel cellhaving an electrolyte with solely oxide or/and proton ionic activity andelectrodes with appropriate catalytic, electronic and ionic activity tofunction in the reduction of air (or oxygen or other oxidant) and theoxidation of hydrogen, hydrocarbon, reformed hydrocarbon or otherappropriate fuel.

PROCESS EXAMPLES

[0040] Some specific examples of tape casting YSZ-based slurries andtape processing will now be given.

[0041] Two sources of YSZ powder have been used. A first powder wasobtained from Pi-Kem Ltd and has the following analysis: TABLE 1 wt %Y₂O₃ 13.62 SiO₂ 0.01 TiO₂ 0.002 Fe₂O₃ 0.003 CaO 0.002 Al₂O₃ 0.25 Na₂O0.003 L O I 0.07 Balance: Zirconia

[0042] The other powder was by Tioxide Ltd; no analysis is available.The powder by Tioxide Ltd was premixed with a binder, but the binder wasremoved by heating at 600° C. overnight.

[0043] Particle size distribution was measured, without deflocculation,by an LS Particle Size Analyser with detection limits of 0.4 m to 2000m. 10 second ultrasonic agitation was performed prior to detection. Thelargest particles detected were 4 m (Pi-Kem Ltd) and 5 m (Tioxide Ltd)and both powders contained particles smaller than 0.4 m. The LS ParticleSize Analyser showed the mode particle size to be 1.43 m (Pi-Kem Ltd)and 1.72 m (Tioxide Ltd).

[0044] A number of dispersing agents were investigated, namelytri-ethanol amine, citric acid, menhaden fish oil, oleic acid, phosphateester (acid form), and polyethylene glycol. Tri-ethanol amine was foundto work well with the Tioxide Ltd product, and phosphate ester (acidform) with the Pi-Kem Ltd product provided the quantity was kept below1.5, preferably 0.05-0.12, g per 10 g of YSZ.

[0045] Tapes were produced using a planetary ball mill and YSZ byTioxide Ltd, with polymethyl methacrylate (PMMA) and polyvinyl butyral(PVB) as binders. The slurry compositions were as follows: TABLE 2Chemical Mass/g (2 dp) a) Binder: PMMA Powder YSZ (Tioxide Ltd) 10.00Solvent Methyl ethyl ketone/ethanol 5.20 (6:4 wt %) DispersantTri-ethanol amine 0.25 Binder PMMA 2.24 Plasticizers Polyethylene glycol(MW300) 1.62 Di-butyl phthalate 1.46 b) Binder: PVB Powder YSZ (TioxideLtd) 10.00 Solvent Methyl ethyl ketone/ethanol 5.20 (6:4 wt %)Dispersant Tri-ethanol amine 0.24 Binder PVB 1.12 PlasticizersPolyethylene glycol (MW300) 0.81 Di-butyl phthalate 0.73

[0046] The tapes produced were flexible, with less binder required whenusing PVB, showing PVB to have better binding properties. For bothtapes, ease of removal was better from a glass carrier than from aMylar® carrier.

[0047] Tapes with PVB binder were noted to be ‘sticky’ and if cominginto contact with themselves were difficult to prise apart. TGA analysisshowed both binders were completely removed by 600° C.

[0048] The tapes were cut into sections and subjected to various firingrates and temperatures. They were fired flat, onto a Safil firing block.

[0049] Slow heating of 1.5° C./min to 600° C., removing theorganic-material, greatly increased tape porosity. The PMMA binder tapehas a larger pore size than the PVB binder tape, due to the higherbinder: powder radio. Both tapes were very brittle.

[0050] Slow heating of 1.5° C./min to 600° C., rapidly heating to 1000°C. (11.5° C./min) and holding at this temperature for 5 hours, againshowed the tapes produced with PMMA binder to be more porous. Comparisonto the tapes heated to 600° C. show a decrease in porosity after thetemperature increase as the tapes contracted. The tapes were lessbrittle after firing at 1000° C., but were still easily broken.

[0051] Tapes were subjected to rapid heating of 11.5° C./min to 1000° C.and holding at this temperature for 5 hours. The tapes are still porous,but interestingly, there is an obvious decrease in porosity for tapesfrom PMMA binder and an increase in porosity for tapes from PVB binderwithout the slow binder removal stage. Again, these tapes were brittle.

[0052] Sintering at 1500° C. for 5 hours after slow binder removalreduced porosity further. The thickness was 124 μm (PVB binder) and theporosity of the PMMA tape to be much higher—reflected by the greaterstrength of the PVB binder tape. Both tapes sintered well. Impuritiesand many holes were present on both tapes. Impurities could be due todust particles, or Si particles picked up from the furnace block.

[0053] A small sample of green tape was rolled according to the geometryin FIG. 4, and fired to 1500° C. Although the above flat tapes showed asmooth surface finish, the rolled tapes did not. This was thought to bedue to too fast a heating rate causing the organic material to bubbleleaving bumps on the surface.

[0054] Intense mixing of the planetary ball mill is thought to haveadverse effects on the binder and further tapes were produced using PVBbinder for YSZ obtained from both Pi-Kem Ltd and Tioxide Ltd, with therotary ball mill.

[0055] Green tapes produced with YSZ (Tioxide Ltd) by rotary andplanetary ball mill were compared. Both ball mills produced a similarhomogenous particle distribution, although more ‘lumps’ are seen in theplanetary ball milled tape. This is possibly due to the more effectivemixing of the planetary ball mill meaning the slurry was mixed for toolong. Mixing of the slurry after binder addition for too long has theeffect of producing less dense tapes, due to the substitution of thedispersant by the binder causing the ‘zipper bag’ effect, where thebinder wraps around a group of particles to form an agglomerate.

[0056] The tapes were heated at 0.8° C./min to 600° C., then to 1000° C.at 1.5° C./min, followed by 3.5° C./min to 1500° C. and sintered at1500° C. The thickness of the tape sintered at 1500° C. was found to bemuch less than the planetary ball milled sample at 82 μm. Halving doctorblade gap height gave a decreased thickness to 45 μm. Both tapes show adecrease in porosity when produced with the rotary ball mill.

[0057] Again, the tapes sintered well. However, localised holes werestill present and impurities were seen in grain boundaries.

[0058] YSZ powder from Pi-Kem Ltd was milled in a rotary ball mill. Theslurry composition was as follows: TABLE 3 Chemical Mass/g(2 dp) PowderYSZ (Pi-Kem Ltd) 20.00 Solvent Methyl ethyl 10.45 ketone/ethanol (6.4 wt%) Dispersant Phosphate Ester (acid 0.21 form) Binder PVB 2.24Plasticizers Polyethylene glycol 1.62 (MW300) Di-butyl phthalate 1.46

[0059] The green tape shows a higher porosity than the green tapeproduced from YSZ (Tioxide Ltd) particles. However, the relativeviscosity of the two slurries, suggests that the YSZ (Pi-Kem Ltd)particles were much better dispersed.

[0060] The tape was shaped into the desired geometry (FIG. 4). They wereheated to 600° C. at 0.5° C./min, then to 1000° C. at 0.8° C./min,followed by heating to 1500° C. at 10° C./min and sintering at 1500° C.for 5 hours. In order to reduce the impurities, an alumina plate wasplaced between the firing block and the samples. Tape thickness wasgreater than the tapes produced by YSZ (Tioxide Ltd)at 76 μm, and thetape was denser. Increase in thickness and density could be explained bydecrease in slurry viscosity.

[0061] The main surface showed fewer impurities, but contained moreholes. This could be attributed to the geometry effectively increasingtape thickness, hence more organic material having to pass through theouter surface.

[0062] It was found that towards the centre of the sintered rolled tapethe layers of tape are in contact with each other and sintered together.However, the outer layer is only in contact with the rest of the samplein small sections.

[0063] PVB was shown to be a more effective binder than PMMA forproduction of green tapes. The smaller quantities of PVB required withrespect to PMMA lead to denser tapes.

[0064] The time-scale used for ball milling (recommended by ‘TapeCasting Theory & Practise’ by Richard E Mistler and Eric R Twiname)shows use of the planetary ball mill produces more porous films.

[0065] The increased number of holes in the rolled tape's surface may bereduced when fired with porous anode and cathode, providing an easierescape route for the organic material.

FURTHER EXAMPLES

[0066] The following examples of slurry formulations have been found tobe better optimised than those presented above. Electrolyte formulationsYSZ 30.00 g Solvent 14.50 g MEK:ethanol 6:4 by weight Dispersant 0.195 gTriton 0.44 Binder  3.36 g PVB Plasticisers  2.43 g polyethyleneglycol 2.19 g di-butylphthalate

[0067] Procedure

[0068] 1. 14 g solvent+powder+dispersant. Ball mill 18 hrs at about 160rpm.

[0069] 2. Add plasticisers+binder+0.5 g solvent. Mix by vibratory mixerfor about 20 min. Ball mill for 4 hrs at about 100 rpm.

[0070] 3. De-air by rolling with no milling media at about 6 rpm forabout 23 hrs.

[0071] 4. Cast on tapecaster TT-1000 from Mistler & Co.

[0072] Speed: 50%

[0073] Doctor blade height: 0.3048 mm (0.012 inch)

[0074] Carrier: Mylar Anode formulations YSZ 5.8633 g weighed correctNiO 7.2570 g {close oversize brace} (by balance) Graphite 4.0984 g to+/− 0.0002 g Solvent 10.125 g MEK:ethanol 6:4 by weight

[0075] YSZ:NiO equivalent to 60:40 of YSZ:Ni by volume on reduction

[0076] NiO+YSZ:graphite is 50:50 by volume

[0077] Procedure

[0078] 1. Ball mill for 18 hours at 160 rpm (ball mill has both rockingand rolling action) with Binder PVB  2.52 g Plasticiserdi-butylphthalate 1.643 g PEG 1.823 g

[0079] Note: no dispersion agent added

[0080] 2. De-air. Ultrasonic agitation 30 min. Vacuum 5 inchHg (belowatmospheric) 5 min.

[0081] Modifications

[0082] The above description refers to electrodes each consisting of asingle uniform layer of sintered material. However, each of theelectrodes could be constituted by composite layers which togetherfulfil the functions of the electrode, namely catalytic performance,electrochemical performance, electronic conduction, and gasdistribution.

[0083] The anode and cathode may each be formed by two or more tapeslaminated together to provide a gradation of function. Also, meshes orribbons may be interspersed between the plural tapes, the meshes ortapes being burnt out during firing to form gas distribution channels.Alternatively the tapes may be appropriately scored using a serrateddoctor-blade to provide such channels. In one example of cathode, aporous layer is formed next to the electrolyte from a mixture of YSZ andlanthanum strontium manganite or other electrode material, and a currentcollection layer with built-in channels is deposited on top of this,made from lanthanum strontium manganate.

[0084] An alternative material to nickel may be used to bridge the gapbetween the high temperature of the fuel cell anode and the lowtemperature of the incoming gas stream, suitably materials based onoxides such as lanthanum chromite. Indeed, the anode itself, or part ofthe anode, may be formed from oxide materials such as lanthanumchromite.

[0085] Summary

[0086] It will be appreciated that the process examples given above areby way of explanation of general principles, rather than preciseexamples of specific formulations. However, from this information theperson skilled in the art will be able to arrive at suitablecompositions and processes for practising the invention, with no morethan routine experimentation.

[0087] Although the preferred form of the invention is the S-shapedlooped coil as shown in FIG. 4, the invention also includes theproduction of flat plate fuel cell components by firing flat tapes.Moreover, by simple rolling up of tapes followed by firing, tubular fuelcell components may be produced.

[0088] Materials other than YSZ may be used, for example scandiastabilised zirconia or scandia+yttria stabilised zirconia, suitably 8-14 mol % scandia+yttria, remainder zirconia; and other materials asdiscussed above.

[0089] Although described with particular reference to fuel cells, theinvention may also be applied to devices for use in electrocatalysis orelectrolysis in a range of gas based processes.

[0090] Other modifications and improvements may be made to the foregoingembodiments within the scope of the invention as defined in the claims.

1. A method of making a component having an anode, a cathode and a solidelectrolyte, the method comprising using tape casting to produce a greentape which is cohesive but flexible, manipulating the green tape toproduce a desired shape and then firing the green tape to produce arigid component; the green tape comprising at least three layers each ofwhich is derived from a respective slurry comprising metal/ceramicparticles dispersed in a carrier liquid; and wherein the step ofmanipulating the green tape to produce a desired shape before beingfired comprises the step of winding the green tape to produce oppositelydirected loops in the centre of the component to form longitudinalchannels separated by a central web, one of the channel being enclosedby an anode surface of the tape and the other by a cathode surface.
 2. Amethod according to claim 1, in which the component is a fuel cellcomponent.
 3. A method according to claim 1, in which the component is acomponent for use in electrolysis or electrocatalysis of gas streams. 4.A method according to claim 1, in which the green tape is formed bycasting at least three slurries one on top of the other and allowing thecarrier liquid to evaporate.
 5. A method according to claim 1, in whichthe green tape is formed by casting at least three separate ribbons andpressing these together, preferably by passing through rollers.
 6. Amethod according to claim 1, in which one or both of the anode and thecathode is formed by plural layers cast from slurries of differingcomposition.
 7. A method according to claim 6, in which there isinterposed between said plural layers a web or mesh of a material whichburns away during firing to leave gas flow passages in the formedelectrode.
 8. A method according to claim 1, in which the carrier liquidcomprises a solvent optionally combined with one or more of adispersant, a binder, and a plasticizer.
 9. A method according to claim1, in which the particles in each of the slurries are based on yttriastabilized zirconia (YSZ).
 10. A method according to claim 8, in whichthe anode slurry comprises particles of YSZ and particles of Ni or NiO,and the cathode slurry comprises particles of YSZ and particles of Srdoped LaMnO₃.
 11. (Cancelled)
 12. (Cancelled)
 13. A method according toclaim 1, in which the green tape is formed with an electrolyte layerwider than the electrode layers and protruding from one side thereof,and in which, before firing, the green tape is wound into a cylindricalform and the protruding electrolyte layer is closed upon itself to forma seal at one end of the component.
 14. A component for use in a fuelcell or an electrochemical device, the component having a generallyelongate tubular form divided by a central web into two channels, one ofthe channels presenting an anode surface to material flowingtherethrough, and the other channel presenting a cathode surface tomaterial flowing therethrough, the component further comprising a solidelectrolyte between said anode and cathode; said component being formedby winding a flexible tape having an anode layer, an electrolyte layerand a cathode layer to produce oppositely directed loops in the centreof the component to form said longitudinal channels separated by saidcentral web.
 15. (Cancelled)
 16. A component according to claim 14, inwhich the flexible tape is a green tape formed by slurry casting andsolvent evaporation; and after being wound the component is fired toproduce a rigid component.
 17. A fuel cell comprising a number ofcomponents as claimed in claim
 14. 18. A fuel cell comprising a numberof components made by the method of claim 1.